This invention relates generally to surface mounting electrical components on printed circuit boards and relates particularly to surface mounting electrical components on printed circuit boards under integrated circuits or other components while furnishing a positive indication of the proper soldering of the electrical components to the printed circuit boards. This indication occurs even though the electrical component rests substantially concealed from sight beneath the integrated circuit package or carrier.
Present memory products such as large capacity--256K--dynamic read only memory devices (DRAMs) often come mounted in close proximity to one another on printed circuit boards. Surface mount technology uses gull wing of J-leads depending from the device package or carriers to connect the device terminals to pads formed on the board using reflow soldering.
Decoupling capacitors, connected between the power supply leads of the devices, often come mounted on the boards with one capacitor reflow soldered to pads under each device. This obtains high density mounting of the memory devices and decoupling capacitors on the boards to reduce costs and reduce otherwise required mounting volume. The capacitor, having dimension of about 0.120.times.0.100.times.0.010 inches, extends into a well or cavity on the underside of each device package or carrier. With a number of leads, such as 18 for a 256K device, depending from the package periphery and soldering to the board, the decoupling capacitor substantially becomes enclosed and screened from sight rendering its visual inspection difficult, usually requiring a microscope. The close mounting of the packages further blocks the view of the capacitor under each package. In a J-lead package, the depending peripheral package material often extends to within 0.025 inch from the seating plane on the board. This leaves as little as 0.025 inch between the board and package and between the leads for visually inspecting the reflow soldering filets on the capacitor, which is only about 0.010 inch thick.
An industry-wide problem exists because automated manufacturing equipment occasionally misplaces the decoupling capacitors from their pads for reflow soldering. This results in the decoupling capacitors occasionally not becoming soldered to their intended pads and remaining loose under the memory device and enclosed by the leads. If inspection fails to catch the loose capacitor or a capacitor wrongly aligned, the capacitor end terminals can touch the device leads and short them electrically. This renders the entire memory circuit card unuseable, and renders the entire system, in which the memory card is mounted, inoperative. Memory cards returned because of loose capacitors cost the customer and marker money and reflect poorly on the manufacturing process. Occasionally, an eventually loose capacitor is held out of position and away from the device leads temporarily by solder flux residue or rests away from the device leads during electrical tests, and accordingly passes inspection by the maker. Later, in use, the capacitor unpredictably becomes loose, shorts together two or more leads and stops the customer's machine.
One solution tried operator visual inspection in addition to electrical testing. This increases cost and remains unreliable because of the small sized parts requiring a microscope and because of the restricted viewing space. Inspection with mechanized vision systems costs hundreds of thousands of dollars but provides only marginal improvement over manual optical inspection. The duplication of these mechanical vision systems and attendant maintenance costs and reduced serviceability of assembly equipment makes this approach to inspecting for failures unfeasible.
Another solution used for stop gap protection vibrates or shakes all assemblies to verify that no decoupling capacitor is loose. This misses, however, capacitors mis-soldered or held temporarily by the solder flux. Alternate inspection procedures include x-raying all cards, which is prohibitively expensive. The x-ray does show a mis-aligned capacitor but lacks sufficient resolution to confirm an acceptable solder joint.
While this problem has been described in conjunction with decoupling capacitors and DRAM devices, like problems exists with capacitors or other components mounted under other types of memory device, such as EEPROMS, EPROMS, ROMS and other integrated circuits.